fix: validation: cast now() to seconds for maxtipage comparison

Since faf44876db, the maxtipage comparison
in IsInitialBlockDownload() has been broken, since the NodeClock::now()
time_point is in the system's native denomination (micrcoseconds).

Without this patch, specifying the maximum allowable -maxtipage
(9223372036854775807) results in a SIGABRT crash.

Co-authored-by: MacroFake <falke.marco@gmail.com>

src/validation.cpp

@@ -1541,7 +1541,7 @@ bool Chainstate::IsInitialBlockDownload() const
     if (m_chain.Tip()->nChainWork < m_chainman.MinimumChainWork()) {
         return true;
     }
-    if (m_chain.Tip()->Time() < NodeClock::now() - m_chainman.m_options.max_tip_age) {
+    if (m_chain.Tip()->Time() < Now<NodeSeconds>() - m_chainman.m_options.max_tip_age) {
         return true;
     }
     LogPrintf("Leaving InitialBlockDownload (latching to false)\n");